A SAN with a plan

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The lowdown

What are they? SANs connect storage and backup devices to servers and clusters of servers to consolidate storage and make it easier to expand and manage the network. They also are useful for disaster recovery and mirroring data center operations over moderate distances.

What networking technology do they use? Fibre Channel is currently the core technology in SANs. It provides up to 2 Gbps of bandwidth for data transfers, at distances up to 6 miles.

How much do they cost? Host adapters, which connect servers to SANs, typically cost about $1,000, and you need one for each connected server. You might also need host adapters to fill slots in switches. Switches themselves cost about $1,000 and up per port.

Must-know info? If you have a small data center, Fibre Channel hubs might be all you need. They link sets of devices into an arbitrated loop topology. Because the maximum transmission rate between individual devices over Fibre Channel is 100 Mbps, multiple devices can run on a Fibre Channel hub without reducing each other's throughput.

If you handle large volumes of data, a switched SAN is probably what you need. Fibre Channel switches create virtual dedicated connections between servers and storage devices, guaranteeing throughput. Switches also support multiple connection types, providing more flexibility in SAN configuration.

Fibre Channel devices put you in touch with large stores of off-site data but don't impact your apps

Data is the grist of any application. The trick has always been getting that data from where it is to where it needs to be without causing bottlenecks in application performance. Today, with the mushrooming storage requirements of distributed applications, managing performance is one of the biggest headaches for system administrators.

High-reliability storage was once limited to mainframe and high-end server systems. But the expanded use of clusters of commodity-level servers and operating systems such as Microsoft Windows NT and 2000 for essential applications has made manageable, redundant and affordable storage solutions a necessity for many organizations.

Now, with recent events reawakening interest in technologies such as off-site mirroring of applications for disaster recovery, the storage management headache has grown more severe. Maintaining dedicated storage for every Web server or application server can become expensive, while sharing disk space over LAN connections can cause major application bottlenecks.

Get connected

The way around the problem is with a storage area network. These dedicated networks connect servers, or clusters of servers, to pools of data storage and archiving devices at higher transfer rates and over greater distances than are practical with conventional storage connection technologies. SANs provide an infrastructure for building redundant, high-availability storage for groups of servers, and they give system administrators a way to expand or upgrade the storage available to servers without having an impact on running applications.

The leading infrastructure technology for SANs is Fibre Channel, which offers up to 2 Gbps of throughput'although the throughput for any single device is limited to 100 Mbps, or 200 Mbps in a full-duplex configuration. Fibre Channel can connect devices at distances up to six miles if the connection is over fiber-optic cabling.

New technologies

A 4-Gbps version of Fibre Channel currently is under development. But Fibre Channel is facing a challenge from other technologies that promise greater transmission speeds, lower costs or both.The technology was devised for point-to-point connections between computer systems and storage devices needing a faster, more robust peripheral link than SCSI could offer.

Then came a loop configuration'Fibre Channel-Arbitrated Loop, or FC-AL'which offered a way to chain multiple devices together. The latest evolution is a switched fabric, with a high-speed switch creating virtual direct connections between devices as required.

A number of data transfer protocols can be used over Fibre Channel connections, including SCSI and IP. This flexibility lets Fibre Channel support multiple storage applications.

One way Fibre Channel technology takes advantage of the flexibility is by tunneling over IP networks'including Ethernet. Tunneling can be especially useful for mirroring data from site to site over a metropolitan network or WAN, either in support of distributed applications or for replication to a backup hot site for disaster recovery or off-site redundancy.

The main problem with Fibre Channel has been compatibility. Fibre Channel standards were loosely adhered to in the past, or extended in proprietary ways, so the result has often been conflicts in hardware from different vendors. Manufacturers recently have worked out a lot of the incompatibilty issues, but now other technologies are posing a challenge to Fibre Channel'most of them from outside the storage-specific networking world.

One of the challengers is Gigabit Ethernet. With 10-Gigabit Ethernet close to becoming a reality, Fibre Channel's throughput edge for SAN applications is rapidly evaporating.

And with faster, less-expensive technology becoming available for short-distance connections to storage devices, other methods will squeeze Fibre Channel on the point-to-point front. For example, the Serial ATA interface standard, released in August but not yet widely adopted by hardware vendors, supports throughput of 1.5 Gbps and a cable length of up to a meter.

Another challenge of sorts for SAN technology is network-attached storage [GCN, Oct. 22, Page 32]. NAS devices are essentially specialized file servers that provide a large amount of network-mountable disk storage to both clients and servers on a LAN. They generally rely on Ethernet for connectivity, and usually support Microsoft Windows and standard network file-sharing protocols such as the Network File System.

NAS or SAN?

In the past, NAS technology offered an easy way to mount more shared disks on a LAN. But with the increasing speed of Ethernet, NAS devices'when combined with high-speed Ethernet switched networks'could challenge current SAN technology. And NAS devices often can be used as a complement to SANs'or in some cases, as an access point to them'because many NAS units rely on Fibre Channel connections to the storage devices themselves.

The main advantage of Fibre Channel SANs in the face of the growing list of alternatives is their lead in support for tools such as storage management software. Fibre Channel SAN switches such as Gadzoox Networks Inc.'s Slingshot series already are certified for enterprise management software including Computer Associates International Inc.'s Unicenter and Tivoli Systems Inc.'s Storage Network Manager module.

SAN switches also support a host of other standard network management tools and interfaces.But in the end, the one thing that can keep Fibre Channel competitive is its decreasing price. As vendors drive down the price per port of Fibre Channel switches, Fibre Channel could still be the SAN of the future for many organizations.

Recent events have heightened IT managers' awareness of the importance of disaster recovery measures. Storage area network technology can be a big aid in ensuring the continued availability of critical data, particularly when combined with high-speed wide area networks.

One example of the resilience of Fibre Channel tunneling over IP was seen outside the government sector, at a company severely affected by the terrorist attacks on Sept. 11.

ESpeed Inc., the technology subsidiary of bond trading company Cantor Fitzgerald LP of New York, lost relatively little data when its data center on the 103rd floor of the World Trade Center's north tower was destroyed in the building's collapse.

The company was able to restore enough of its electronic trading applications in time for the opening of the U.S. bond market less than two days later. The reason: a combination of Fibre Channel tuneling and software data replication between the primary data center at One World Trade Center and an off-site data center in northern New Jersey. Data was also replicated to a data center overseas.

Data protection

SAN technology also made it possible to scale up the backup media resources at the New Jersey facility from tapes kept off-site, in order to restore nearly 100 servers in two days.

The majority of government agencies don't have the same operational requirements as the bond market'no downtime ever'but their similar requirement for redundancy is a leading reason for SAN deployment.

Supercomputing programs and defense and law enforcement databases need to be resilient when a major component fails. But managers also need to protect a whole set of computing assets against communications outages or other kinds of interruptions, not the least of which are natural or manmade disasters.

Innovations such as extending Fibre Channel networks through the use of repeaters and Fibre Channel tunneling over IP can disperse IT assets without compromising the performance of applications. Let's be realistic: That's a feature that will continue to be in high demand.